The invention relates to a tread pattern for a tread of a tire, and more particularly to a tread pattern which evolves as the tread becomes worn; the invention also relates to a mold for molding such an evolving tread pattern.
A tread of a tire is generally formed by at least one elastomer and is provided with a tread pattern formed of elements in relief (such as ribs or blocks) separated from each other in the circumferential direction and/or in the transverse direction by grooves having cross-sections which have widths typically greater than 2 mm and depths at most equal to the thickness of the tread.
A tire traveling over long distances results in gradual wear of the tread of said tire up to a maximum level of wear which determines what is called the wear life of the tire, which requires either the replacement of the tire or the renovation of the tread thereof by recapping and putting on a new tread.
The elements in relief of a tread play a fundamental role as far as the adhesion of the tire is concerned, both in the transverse direction and in the circumferential direction, and very particularly when traveling on wet and/or snowy ground. In fact, the ridges of the elements in relief, by cutting the film of water present on the surface of the road, make it possible to keep the tread in contact with the road; secondly, the grooves act as preferred passages for evacuating the water in this case.
Obtaining a good level of performance of a tread pattern of a tire also lies in the conformity between said tread pattern of the new tire and the rigidity of the tread. xe2x80x9cRigidityxe2x80x9d is understood, among other things, to mean the rigidity of said tread when it is subjected both to loading forces (compressive forces) in the region affected by the contact with the road, for example, and to forces tangential to said region (these latter forces are comparable with shearing forces).
For a given nature of the rubber mix forming the tread, this loading resistance under compressive and shearing stresses depends on the dimensions of the contact surfaces of the elements in relief in contact with the ground and on the height of said elements, that is to say the depth of the grooves defining said elements. The different performances in terms of wear, behavior on dry or wet ground, or of adhesion of a tread of a tire are highly dependent on the rigidity of this tread in the zone affected by the loading.
Furthermore, and in order to increase the number of rubber ridges in contact with the road, a greater or lesser number of incisions or slits is frequently produced in the elements in relief, the width of which incisions or slits is very much less than the width of the grooves defining the elements in relief. The width of an incision, which is variable according to the dimension of the tire in question, is generally less than 2 mm. As a general rule, the incisions have the same depth as the grooves, but they may also affect only the surface part of the tread so as not to reduce too far the initial rigidity of said tread and thus to achieve an acceptable compromise.
The presence of a greater or lesser number of incisions modifies in proportion the rigidity of the elements in relief provided with such incisions, and consequently the rigidity of the tread is affected to a greater or lesser extent thereby.
The assembly formed by these circumferential and/or transverse grooves associated with the incisions produced in the elements in relief of the pattern of a tread imparts to the new tire provided with said tread a level of performance which is entirely satisfactory to the user, whatever the type of roads and/or the climatic conditions encountered.
Nevertheless, the reduction in the thickness of the tread resulting from the progressive wear of the tire results in a reduction in the depth of the grooves and hence in a reduction in the height of the elements in relief. Furthermore, this reduction in the height of the elements in relief results in an increase in the rigidity of each element, and consequently of said tread. Associated with this wear, there is generally noted a loss of effectiveness of the tread pattern after a greater or lesser fraction of the life of the tire. To take account of this, provision may be made to achieve a compromise on the tire when new by providing it with a tread pattern, the optimum point of operation of which is only achieved after a level of wear of its tread.
Another means making it possible to achieve satisfactory operation on the new tire and after a given amount of wear of the tread consists, as described in European Patent 0,378,090, in providing the tread of a new tire with a plurality of incisions having in cross-section a trace, formed of a first rectilinear part extending radially to the inside of the tire over a depth of between 35% and 55% of the total depth of the trace, this first part being divided into at least two branches extending radially to the inside of the tire over the remaining depth such that the number of incisions is multiplied by a coefficient of at least 1,5 starting from the inner end of the first rectilinear part of the trace.
Although this solution achieves good results, it is however not totally satisfactory, because, since the wear of the tread involves a reduction in the volume determined by the grooves and possibly the incisions if present, there results a reduction in the ability of the grooves to evacuate the water present on the road in the case of traveling on a wet road. To overcome this disadvantage, it is known, for example in the field of heavy-vehicle tires, to regenerate the tread pattern after partial wear of the tread by removing material to form new grooves according to a pre-established trace.
The problem on which the invention is based is to produce a tread of a tire which is optimized from the point of view of the adhesion and behavior performance (that is to say, ensuring perfect control of the road-holding of a vehicle equipped with tires provided with said tread pattern), whether in the initial state or during the life of the tire without this adversely affecting the performance of said tire in terms of its wear life.
Another object of the invention is to provide a tread pattern imparting to a tread a rigidity in the region affected by the contact which is adapted according to the successive states of wear of said tread.
According to the invention, there is proposed a tread of thickness E for a tire, said tread having a running surface, limited transversely by lateral edges, said tread being provided when new with a tread pattern comprising at least a plurality of rubber blocks arranged circumferentially on at least one of its lateral edges, said rubber blocks being separated from each other by grooves oriented virtually transversely and separated from the tread pattern motifs located axially to the inside by grooves oriented mainly circumferentially, the tread being characterized in that it comprises on at least one of its edges comprising a plurality of rubber blocks when new and radially to the inside of each of said blocks:
at least one channel of average width at least equal to the width of the transverse grooves, each channel having an average trace identical to the average trace of said transverse grooves and being intended to form a groove opening radially towards the outside of the tread after wear of the tread of between 15% and 45% of the thickness E of said tread, each channel extending within the remaining thickness of the tread, and
at least one incision, of low width compared with the average width of each channel, having, in a radial projection on the running surface of the tread when new, an average transverse orientation identical or virtually identical to that of the channel(s) projected on the same surface, each incision opening radially to the running surface at the latest at the time of the appearance of a channel on said running surface and extending, in the thickness of the tread, over the entire depth of the adjacent channel(s) on the same level in the thickness of the tread.
It is judicious for the channels intended to form new grooves to extend in depth at least as far as the maximum level of wear of the tread, so as to maintain good drainage during the entire time of use of the tire provided with a tread according to the invention.
Another object of the invention is to propose a mold for manufacturing a tire for molding such a tread, said mold firstly permitting the molding of a plurality of motifs located radially beneath the running surface of said tread and extending in a substantially transverse direction, and secondly the demolding of the tire using the forces exerted by the material constituting the tread on said mold, so as to reduce the overall molding force and to facilitate the latter.
Usually, a tire mold comprises several parts which, once assembled, define a molding surface corresponding to the outer surface of the tire to be molded. One type of known mold comprises a part forming a ring, intended to mold the tread of a tire, and two parts forming shells intended to mold the sidewalls of said tire. The ring molding the tread is composed of a plurality of sectors, each sector, being able to be displaced in a substantially radial direction of molding/demolding, is provided with a mold fitting composed of a plurality of elements in relief for molding motifs in said tread. Using this known technique, the elements in relief have to have geometries which permit demolding after vulcanization of the tire, that is to say the extraction of said elements from the tread. Consequently, elements in relief having undercut parts are difficult to demold using this technique. Likewise, this technique does not make it possible to mold elements which do not open on to the surface of the tread when new.
For molding channels beneath the running surface of the tread of a new tire, it is known, in particular in U.S. Pat. No. 1,733,064, to produce a plurality of holes in a shell of a tire mold so as to permit the introduction of needles into the rubber forming the tread of a tire once the latter has been molded. Furthermore, means external to said mold are provided for forcing said needles to penetrate the rubber, these same means then serving to cause the needles to emerge again before proceeding to demould the tire by opening the different parts constituting the mold. This mold is particularly complex to produce, and requires at least one shell to be pierced; in the case of a modification, for example of the number of needles, it is necessary to renovate another shell, which is expensive. Furthermore, the presence of holes on the shell involves greater or lesser penetration of the rubber thereinto, which results, after molding, in the presence of molding defects on the sidewalls of the tire.
To solve the problems mentioned above, there is proposed a mold for molding a tire provided with a tread extending axially via the sidewalls as far as the bead zones, said mold comprising two shells provided with molding surfaces each for molding a tire sidewall and a structure forming a ring concentric to the two shells for molding the tread of the tire. The ring and the shells forming the mold according to the invention, in the closed position of the mold, are in contact via their lateral ends so as to define the total molding surface of a tire.
The mold according to the invention is characterized in that:
at least one shell is formed of at least two annular shell parts concentric to each other, such that the molding surfaces of said parts, in the molding configuration, are in the extension one of the other for molding an entire sidewall,
at least one of the shell parts is provided with at least a plurality of elements in relief projecting onto the molding surface of said shell part, said elements in relief of the same shell part having virtually the same geometry and the same orientation defined as being the direction between the point of anchoring of an element on the shell part and the end of said axially outermost element of said shell part,
each of the shell parts bearing elements in relief is mounted to be mobile in the circumferential direction relative to the other parts of the same shell, so as to permit the rotation of each of said parts relative to the other parts of the same shell at least under the action of the forces exerted by the rubber mixes on the elements in relief during demoulding, with the aim of facilitating the demoulding of the tire.
In this manner, it is possible to reduce the forces necessary for demolding the elements in relief borne by each shell part, since under the action of the reaction forces produced by the vulcanized rubber mixes during the demolding on said elements, each part will rotate relative to the other parts of the same shell.
A mold according to the invention makes it possible, in simple manner, to effect molding and demolding of tires comprising tread pattern motifs opening on to at least one edge of the tread, said motifs having an orientation which is overall different from the transverse direction of the tire. The mold according to the invention also makes it possible to mold tread pattern motifs with elements in relief having highly non-demoldable geometries (that is to say, geometries such that the molding in a non-elastically deformable material cannot be followed by demolding without degrading either the elements in relief or the molded material).
A mold according to the invention may comprise at least one shell formed by the assembly of several annular parts concentric to one another, of which at least two of said parts are circumferentially mobile relative to a fixed part of said shell to permit, for example, the production of several levels of tread pattern in a tread (an arrangement which is particularly advantageous in the case of tires for heavy vehicles having treads of a thickness which is relatively greater than that of passenger car tires). Advantageously, the shell parts bearing elements in relief are mobile independently of each other.